The present invention relates to a method for removing CO2 (carbon dioxide) from a combustion exhaust gas. More specifically, it relates to a method for removing CO2 from a combustion exhaust gas under atmospheric pressure by the use of a specific mixed aqueous solution containing an amine.
In recent years, a greenhouse effect by CO2 is indicated as one cause of the warming phenomenon of the earth, and its prompt resolution is globally required in order to protect earth circumstances. The generation sources of CO2 extend in active fields of all humans in which fossil fuels are burned, and there is a tendency that the discharge regulation of CO2 will be further tightened in the future. Thus, for power generation facilities such as thermoelectric power plants in which a large amount of the fossil fuel is used, there are energetically researched a method for removing and recovering CO2 from a combustion exhaust gas by bringing the combustion exhaust gas coming from a boiler into contact with an aqueous alkanolamine solution or the like, and a method for storing the recovered CO2 without discharging it into the atmosphere.
Examples of the alkanolamine include monoethanolamine, diethanolamine, triethanolamine, dimethyldiethanolamine, diisopropanolamine and diglycolamine, and in general, monoethanolamine (abbreviated to xe2x80x9cMEAxe2x80x9d) is preferably used.
However, even if the above-mentioned aqueous alkanolamine solution typified by MEA is used as an absorbing solution for absorbing/removing CO2 from a combustion exhaust gas, the effect of the alkanolamine is not always satisfactory in view of an amount of absorbed CO2 per predetermined amount of the aqueous alkanolamine solution having a predetermined concentration, an amount of absorbed CO2 per unit amine mole of the aqueous alkanolamine solution having a predetermined concentration, an absorption rate of CO2 at a predetermined concentration, heat energy required to recover the aqueous alkanolamine solution after the absorption, and the like.
In the meantime, for the separation of an acidic gas from various mixed gases by the use of an amine compound, many techniques are known.
Japanese Patent Application Laid-open No. 100180/1978 discloses a method for removing an acidic gas which comprises bringing a usually gaseous mixture into contact with an amine-solvent liquid absorbent comprising
(1) an amine mixture comprising at least 50 mole % of a steric hindrance amine constituting a part of a ring and having at least one secondary amino group bonded to either of a secondary carbon atom or a tertiary carbon atom or a primary amino group bonded to the tertiary carbon atom, and at least about 10 mole % of the tertiary amino-alcohol, and
(2) a solvent for the above-mentioned amine mixture which functions as a physical absorbent for the acidic gas. Usable examples of the steric hindrance amine include 2-piperidine ethanol [2-(2-hydroxyethyl)-piperidine] and 3-amino-3-methyl-1-butanol, and a usable example of the tertiary amino-alcohol is 3-dimethylamino-1-propanol. Furthermore, an example of the solvent is a sulfoxide compound which may contain water in an amount of 25% by weight or less, and an example of a gas to be treated is xe2x80x9ca usually gaseous mixture containing carbon dioxide and hydrogen sulfide at high concentrations, for example, 35% Of CO2 and 10-12% of H2Sxe2x80x9d on page 1, left upper column of the same gazette. In the undermentioned examples, CO2 itself is used.
In Japanese Patent Application Laid-open No. 71819/1986, there is described a composition for the scraping of an acidic gas which contains a non-aqueous solvent such as a steric hindrance amine or sulfolane. As an example of the primary monoamino alcohol of the steric hindrance, 2-amino-2-methyl-1-propanol (abbreviated to AMP) is exemplified and used. In examples, CO2 and nitrogen as well as CO2 and helium are used. Furthermore, as absorbents, an aqueous solution of an amine and potassium carbonate, and the like are used. The use of water is also referred to. In addition, this gazette describes the advantage of the steric hindrance amine in the absorption Of CO2 by reaction formulae.
In Chemical Engineering Science, Vol. 41, No. 4, pp. 997-1,003, there is disclosed a carbon dioxide gas absorption behavior of an aqueous AMP solution which is a hindered amine. As gases to be absorbed, CO2 and a mixture of CO2 and nitrogen at atmospheric pressure are used.
Chemical Engineering Science, Vol. 41, No. 4, pp. 405-408 has reported absorption rates of an aqueous solution of a hindered amine such as AMP and an aqueous solution of a straight-chain amine such as MEA to CO2 and H2S in the vicinity of ordinary temperature. According to this report, a large difference is not present between both the aqueous solutions, in the case that the partial pressure of CO2 is 1 atm and the concentrations of the aqueous solutions are from 0.1-0.3 mole. However, it is apparent that when the concentrations of the aqueous solutions are 0.1 mole and the partial pressure of CO2 is decreased to 1, 0.5 and 0.05 atm, the absorption rate of AMP deteriorates more largely than that of MEA at 0.05 atm.
U.S. Pat. No. 3,622,267 discloses a technique in which an aqueous mixture containing methyldiethanolamine and monoethylmonoethanolamine is used to purify a highpartial pressure CO2 contained in a synthetic gas such as a partially oxidized gas of a crude oil or the like, for example, a synthetic gas containing 30% of CO2 at 40 atm.
German Laid-open Patent No. 1,542,415 discloses a technique in which a monoalkylalkanolamine or the like is added to a physical or chemical absorbent in order to improve the absorption rate of CO2, H2S and COS. Similarly, German Laid-open Patent No. 1,904,428 discloses a technique in which monomethylethanolamine is added for the purpose of improving the absorption rate of methyldiethanolamine.
U.S. Pat. No. 4,336,233 discloses a technique in which, for the purification of a natural gas, a synthetic gas or a gasified coal gas, a 0.81-1.3 mole/liter aqueous piperazine solution is used as a wash liquid, or piperazine is used in the state of an aqueous solution together with a solvent such as methyldiethanolamine, triethanolamine, diethanolamine or monomethylethanolamine as a wash liquid;
Similarly, Japanese Patent Application Laid-open No. 63171/1977 discloses a CO2 absorbent obtained by adding piperazine or a piperazine derivative such as hydroxyethylpiperazine as an accelerator to a tertiary alkanolamine, a monoalkylalkanolamine or the like.
As described above, a method for efficiently removing CO2 from a combustion exhaust gas has been heretofore desired. In particular, in the case that the combustion exhaust gas is treated with an aqueous solution containing a CO2 absorbent at a certain concentration, it is an urgent serious problem to select an absorbent which is capable of absorbing a large amount of CO2 per unit mole of the absorbent and absorbing a large amount of CO2 per unit volume of the aqueous solution and which has a high absorption rate. Furthermore, another requirement of the absorbent is to permit the separation of CO2 and the recovery of the absorbing solution with a small amount of heat energy, after the absorption of CO2. Above all, it is desired to improve the absorption rate of the absorbent having a large CO2 absorption power but a low absorption rate.
In view of the above-mentioned problems, the present inventors have intensively investigated an absorbent for use in the removal of CO2 from a combustion exhaust gas. As a result, they have found that the employment of a specific hindered amine is particularly effective, and thus the present invention has now been completed.
That is, the present invention is directed to a method for removing carbon dioxide from a combustion exhaust gas which comprises the step of bringing the combustion exhaust gas under atmospheric pressure into contact with an aqueous solution of a hindered amine (exclusive of an amine having two or more amino groups) selected from the group consisting of:
(A) a compound having an alcoholic hydroxyl group and a primary amino group, said primary amino group being bonded to a tertiary carbon atom having two unsubstituted alkyl groups,
(B) a compound having an alcoholic hydroxyl group and a secondary amino group, said secondary amino group having an N atom bonded to a group having a chain of 2 or more carbon atoms inclusive of a bonded carbon atom,
(C) a compound having an alcoholic hydroxyl group and a tertiary amino group, at least two groups bonded to said tertiary amino group having a chain of 2 or more carbon atoms inclusive of a bonded carbon atom, respectively, two of the groups bonded to said tertiary amino group being unsubstituted alkyl groups, and
(D) a 2-substituted piperidine having a hydroxyl group-substituted alkyl group at the 2-position.
In the present invention, the above-mentioned hindered amine is particularly preferably selected from the group consisting of 2-amino-2-methyl-1-propanol, 2-(methyl-amino)-ethanol, 2-(ethylamino)-ethanol, 2-(diethylamino)-ethanol and 2-(2-hydroxyethyl)-piperidine.
Every hindered amine which can be used in the present invention has an alcoholic hydroxyl group in its molecule. It is preferred that one alcoholic hydroxyl group is present in the hindered amine molecule. Further more, the molecular weight of the hindered amine is preferably 150 or less from the viewpoint of CO2 absorption power per unit amount of the solution at a predetermined concentration.
One of the hindered amines which can be used in the present invention is (A) a compound having an alcoholic hydroxyl group and a primary amino group, said primary amino group being bonded to a tertiary carbon atom having two unsubstituted alkyl groups. In this (A), the unsubstituted alkyl groups may be mutually identical or different, and examples of the unsubstituted alkyl groups include a methyl group, an ethyl group and a propyl group, but both of the unsubstituted alkyl groups are preferably the methyl groups. Typical examples of this (A) include 2-amino-2-methyl-1-propanol, 3-amino-3-methyl-2-pentanol, 2,3-dimethyl-3-amino-1-butanol, 2-amino-2-ethyl-1-butanol, 2-amino-2-methyl-3-pentanol, 2-amino-2-methyl-1-butanol, 3-amino-3-methyl-1-butanol, 3-amino-3-methyl-2-butanol, 2-amino-2,3-dimethyl-3-butanol, 2-amino-2,3-dimethyl-1-butanol and 2-amino-2-methyl-1-pentanol, and above all, 2-amino-2-methyl-1-propanol (AMP) is preferable.
Another amine of the hindered amines which can be used in the present invention is (B) a compound having an alcoholic hydroxyl group and a secondary amino group, said secondary amino group having an N atom bonded to a group having a chain of 2 or more carbon atoms inclusive of a bonded carbon atom. In this (B), the group having the chain of 2 or more carbon atoms inclusive of the bonded carbon atom is usually a hydroxy group-substituted alkyl group of 2-5 carbon atoms, preferably an alkyl group of 2-3 carbon atoms which may be substituted by the hydroxyl group. Typical examples of this (B) include 2-(ethylamino)-ethanol, 2-(methylamino)-ethanol, 2-(propylamino)-ethanol, 2-(isopropylamino)-ethanol, 1-(ethylamino)-ethanol, 1-(methylamino)-ethanol, 1-(propylamino)-ethanol and 1-(isopropylamino)-ethanol, and above all, 2-(ethylamino)-ethanol (hereinafter abbreviated to xe2x80x9cEAExe2x80x9d) and 2-(methyl-amino)-ethanol (hereinafter abbreviated to xe2x80x9cMAExe2x80x9d) are preferably used.
Still another amine of the hindered amines which can be used in the present invention is (C) a compound having an alcoholic hydroxyl group and a tertiary amino group, at least two groups bonded to said tertiary amino group having a chain of 2 or more carbon atoms inclusive of a bonded carbon atom, respectively, two of the groups bonded to said tertiary amino group being unsubstituted alkyl groups. In this (C), the two unsubstituted alkyl group may be mutually identical or different, and examples of the unsubstituted alkyl groups include a methyl group, an ethyl group, a propyl group and an isopropyl group. Typical examples of this (C) include 2-(dimethylamino)-ethanol, 2-(diethylamino)-ethanol, 2-(ethylmethylamino)-ethanol, 1-(dimethylamino)-ethanol, 1-(diethylamino)-ethanol, 1-(ethylmethylamino)-ethanol, 2-(diisopropylamino)-ethanol, 1-(diethylamino)-2-propanol and 3-(diethylamino)-1-propanol, and above all, 2-(diethylamino)-ethanol (herein after abbreviated to xe2x80x9cDEAExe2x80x9d) is preferably used.
Still another amine of the hindered amines which can be used in the present invention is (D) a 2-substituted piperidine having a hydroxyl group-substituted alkyl group at the 2-position. Typical examples of the 2-substituted piperidine include 2-(hydroxymethyl)-piperidine, 2-(2-hydroxyethyl)-piperidine and 2-(1-hydoxymethyl)-piperidine, and above all, 2-(2-hydroxyethyl)-piperidine (hereinafter abbreviated to xe2x80x9cHEPxe2x80x9d) is preferable.
The hindered amines for use in the present invention selected from the above-mentioned groups can be utilized singly or in the form of a mixture.
The concentration of the aqueous hindered amine solution which can be used as an absorbing solution is usually from 25 to 65% by weight, depending upon the kind of hindered amine. The temperature of the aqueous hindered amine solution at the time of the contact with the combustion exhaust gas is usually in the range of 30 to 70xc2x0 C.
If necessary, a corrosion inhibitor, a hindered amine aging inhibitor and the like can be added to the aqueous hindered amine solution. As these inhibitors, conventionally usable inhibitors can be used.
In this connection, the expression xe2x80x9cunder atmospheric pressurexe2x80x9d in the present invention covers a pressure range including the vicinity of the atmospheric pressure which permits the function of a blower or the like for feeding the combustion exhaust gas.
In view of the above-mentioned problems, the present inventors have intensively investigated an absorbent for use in the removal of CO2 from the combustion exhaust gas, and as a result, they have found that the utilization of a mixture obtained by mixing a specific amine compound (X) with a relatively small amount of a specific amine compound (Y) is particularly effective to improve the absorption rate of the specific amine compound (X). In consequence, the present invention has now been achieved.
That is, the present invention is directed to a method for removing carbon dioxide from a combustion exhaust gas which comprises the step of bringing the combustion exhaust gas under atmospheric pressure into contact with a mixed aqueous solution of 100 parts by weight of an amine compound (X) selected from the group consisting of (A) a compound having one alcoholic hydroxyl group and a primary amino group in its molecule, said primary amino group being bonded to a tertiary carbon atom having two unsubstituted alkyl groups, (B) a compound having one alcoholic hydroxyl group and a tertiary amino group in its molecule, at least two groups bonded to said tertiary amino group having a chain of 2 or more carbon atoms inclusive of a bonded carbon atom, respectively, two of the groups bonded to said tertiary amino group being unsubstituted alkyl groups, and (C) diethanolamine; and 1-25 parts by weight of an amine compound (Y) selected from the group consisting of (D) piperazine, (E) piperidine, (F) morpholine, (G) glycine, (H) 2-piperidinoethanol, and (I) a compound having one alcoholic hydroxyl group and a secondary amino group in its molecule, said secondary amino group having an unsubstituted alkyl group of 3 or less carbon atoms and an N atom bonded to a group having a chain of 2 or more carbon atoms inclusive of a bonded carbon atom.
As a particularly preferable embodiment of the present invention, there can be recited a method for removing CO2 from a combustion exhaust gas which comprises the step of bringing the combustion exhaust gas under atmospheric pressure into contact with a mixed aqueous solution of 100 parts by weight of an amine compound, as the above-mentioned amine compound (X), selected from the group consisting of 2-amino-2-methyl-1, 3-propanediol, 2-amino-2-methyl-1-propanol, 2-amino-2-ethyl-1, 3-propanediol, t-butyldiethanolamine and 2-amino-2-hydroxymethyl-1,3-propanediol; and 1-25 parts by weight of an amine compound, as the above-mentioned amine compound, selected from the group consisting of piperazine, piperidine, morpholine, glycine, 2-methylaminoethanol, 2-piperidineethanol and 2-ethylaminoethanol.
The combination of the specific amine compounds (X) and (Y) which can be used in the present invention are as described above. However, one amine compound (X) may be combined with one amine compound (Y), or alternatively, one of either group of the amine compounds (X) and (Y) may be combined with two or more of the other group.
One of the amine compounds which can be used in the present invention is (A) a compound having one alcoholic hydroxyl group and a primary amino group in its molecule, said primary amino group being bonded to a tertiary carbon atom having two unsubstituted alkyl groups. In this (A), the unsubstituted alkyl groups may be mutually identical or different, and their examples include a methyl group, an ethyl group and a propyl group. Preferably, both of the unsubstituted alkyl groups are the methyl groups. Typical examples of this (A) include 2-amino-2-methyl-1-propanol, 3-amino-3-methyl-2-pentanol, 2,3-dimethyl-3-amino-1-butanol, 2-amino-2-ethyl-1-butanol, 2-amino-2-methyl-3-pentanol, 2-amino-2-methyl-1-butanol, 3-amino-3-methyl-1-butanol, 3-amino-3-methyl-2-butanol, 2-amino-2,3-dimethyl-3-butanol, 2-amino-2,3-dimethyl-1-butanol and 2-amino-2-methyl-1-pentanol. Above all, 2-amino-2-methyl-1-propanol (AMP) is preferable.
Another compound of the amine compounds which can be used in the present invention is (B) a compound having one alcoholic hydroxyl group and a tertiary amino group in its molecule, at least two groups bonded to the tertiary amino group having a chain of 2 or more carbon atoms inclusive of a bonded carbon atom, respectively, two of the groups bonded to the tertiary amino group being unsubstituted alkyl groups. In this (B), the two unsubstituted alkyl group may be mutually identical or different, and their examples include a methyl group, an ethyl group, a propyl group and an isopropyl group. Typical examples of this (B) include 2-(dimethylamino)-ethanol, 2-(diethyl-amino)-ethanol, 2-(ethylmethylamino)-ethanol, 1-(dimethyl-amino)-ethanol, 1-(diethylamino)-ethanol, 1-(ethylmethylamino)-ethanol, 2-(diisopropylamino)-ethanol, 1-(diethylamino)-2-propanol and 3-(diethylamino)-1-propanol, and above all, 2-(diethylamino)-ethanol (hereinafter abbreviated to xe2x80x9cDEAExe2x80x9d) are preferable.
Still another compound of the amine compounds which can be used in the present invention is (I) a compound having one alcoholic hydroxyl group and a secondary amino group in its molecule, the secondary amino group having an unsubstituted alkyl group of 3 or less carbon atoms and an N atom bonded to a group having a chain of 2 or more carbon atoms inclusive of a bonded carbon atom. In this (I), an example of the chain of 2 or more carbon atoms inclusive of the bonded carbon atom is usually an hydroxyl group-substituted alkyl group of 2-5 carbon atoms, preferably an hydroxyl group-substituted alkyl group of 2-3 carbon atoms. Typical examples of this (I) include 2-(ethylamino)-ethanol, 2-(methylamino)-ethanol, 2-(propylamino)-ethanol, 2-(isopropylamino)-ethanol, 1-(ethylamino)-ethanol, 1-(methylamino)-ethanol, 1-(propylamino)-ethanol and 1-(isopropylamino)-ethanol, and above all, 2-(ethylamino)-ethanol and 2-(methylamino)-ethanol (hereinafter abbreviated to xe2x80x9cMAExe2x80x9d) are preferably used.
With regard to a mixing ratio of the amine compounds (X) and (Y), the amine compound (Y) is in the range of 1 to 25 parts by weight, preferably in the range of 1 to 10 parts by weight, based on 100 parts by weight of (X) in the case that the amine compound (X) comprises (A) and/or (B). Furthermore, the amine compound (Y) is in the range of 1 to 25% by weight, preferably in the range of 10 to 25% by weight, based on 100 parts by weight of (X) in the case that the amine compound (X) comprises (C) diethanol. The concentration of the amine compound (X) in the mixed aqueous solution (which may be called an absorbing solution) is usually from 15 to 65% by weight, depending upon the kind of (X). The temperature of the mixed aqueous solution at the time of the contact with the combustion exhaust gas is usually in the range of 30 to 70xc2x0 C.
If necessary, a corrosion inhibitor, a hindered amine aging inhibitor and the like can be added to the mixed aqueous solution which can be used in the present invention.
In this connection, the expression xe2x80x9cunder atmospheric pressurexe2x80x9d in the present invention covers a pressure range including the vicinity of the atmospheric pressure which permits the function of a blower or the like for feeding the combustion exhaust gas.